Server virtualization in data centers is becoming widespread. In general, server virtualization describes a software abstraction that separates a physical resource and its use from the underlying physical machine. Most physical resources can be abstracted and provisioned as virtualized entities. Some examples of virtualized entities include the central processing unit (CPU), network input/output (I/O), and storage I/O.
Virtual machines (VM), which are a virtualization of a physical machine and its hardware components, play a central role in virtualization. A virtual machine typically includes a virtual processor, virtual system memory, virtual storage, and various virtual devices. A single physical machine can host a plurality of virtual machines. Guest operating systems execute on the virtual machines, and function as though executing on the actual hardware of the physical machine.
A layer of software provides an interface between the virtual machines resident on a physical machine and the underlying physical hardware. Commonly referred to as a hypervisor or virtual machine monitor (VMM), this interface multiplexes access to the hardware among the virtual machines, guaranteeing to the various virtual machines use of the physical resources of the machine, such as the CPU, memory, storage, and I/O bandwidth.
Typical server virtualization implementations have the virtual machines share the physical network interface (aka, network adapter, network interface card, or NIC) of the physical machine for performing external network I/O operations. The hypervisor typically provides a virtual switched network (called a vswitch) that provides interconnectivity among the virtual machines on a single physical machine. The vswitch interfaces between the NIC of the physical machine and the virtual NICs (vNICs) of the virtual machines, each virtual machine having one associated vNIC. In general, each vNIC operates like a physical NIC, being assigned a media access control (MAC) address that is typically different from that of the physical NIC. The vswitch performs the routing of packets to and from the various virtual machines and the physical NIC.
Advances in network I/O hardware technology have produced multi-queue NICs that support network virtualization by reducing the burden on the vswitch and improving network I/O performance. A multi-queued NIC can be provisioned into multiple virtual NICs and can be configured as multiple NICs within an operating system. Generally, multi-queue NICs assign transmit and receive queues to each virtual machine. The NIC places outgoing packets from a given virtual machine into the transmit queue of that virtual machine and incoming packets addressed to the given virtual machine into its receive queue. The direct assignment of such queues to each virtual machine thus simplifies the handling of outgoing and incoming traffic.
Another advance in network I/O hardware technology is a physical network interface known as a converged network adapter (CNA). In general, a CNA combines the data networking of a NIC with storage networking; a single physical network interface can send and receive network data packets and storage data packets. Each CNA can have multiple virtual network interfaces or multiple instances of physical network interfaces implemented in a single physical device.
In the typical virtualization environment, however, I/O has become a performance bottleneck. Solutions to the I/O bottleneck, however, tend to overlook implications stemming from incorporating into the physical network interface network-switching functionality that increasingly overlaps the network-switching functionality of the fabric.